This invention relates to the photolithographic, or masking, process in the manufacture of semiconductors, and more particularly to the coating of a semiconductor wafer with photoresist.
The manufacture of semiconductors usually includes several masking steps to form patterns on the semiconductor wafer for subsequent etching and diffusion, metalization or other steps. In a typical masking, or photolithographic process, the following steps occur:
(1) the wafer is coated with photoresist film; PA0 (2) the wafer is baked to dry the film and enhance its adhesion to the wafer; PA0 (3) the film is exposed to ultraviolet light, e-beams or x-rays through a photomask; PA0 (4) the exposed film is removed with a developer solution; PA0 (5) the wafer is baked to harden the photoresist pattern; and PA0 (6) the wafer is etched with etchant where exposed film was removed.
In the first (coating) step, producing a thin and uniform film is critical to the remaining steps. If the film is too thick, it is difficult to obtain sharp patterns when the film is exposed. If the film is too thin, the photoresist may not adequately mask the whole material or it may form pinholes. If the film is not uniform, the removal of exposed film may be uneven and can lead to variable etch line widths and line discontinuities.
There are many methods of coating a wafer with photoresist, such as flow coating, roller coating, spray coating, dip coating, and spin coating (also known as whirl coating). Spin coating is probably the most widely used coating method in the microelectronics industry today. In spin coating, a piece of substrate to be coated (e.g. a silicon wafer) is mounted on a disc centered on a shaft. An amount of photoresist is deposited on the center of the wafer. The disc is spun about the shaft at a constant rpm after a high initial acceleration rate. The centrifugal force generated by the spinning causes the photoresist coating to move out to the edges of the wafer and produces a thin coating on the surface of the wafer. Because the magnitude of the centrifugal force on the disc is proportional to the radius, the farther a point on the disc is away from the center of the disc, the stronger the centrifugal force at that point is. Although the surface tension of the liquid compensates to a large extent for the difference in centrifugal force, nevertheless, the thickness of the coating decreases as distance from the center increases, resulting in a non-uniform film.